This invention relates to a method and for determining the position of a mobile terminal operating in environments with internal and external positioning systems.
The position of a mobile terminal can be determined by: (1) an internal positioning apparatus such as a GPS receiver built into the mobile terminal that receives Global Positioning System (xe2x80x9cGPSxe2x80x9d) radio signals transmitted from GPS satellites; and (2) an external positioning apparatus such as a cellular positioning system that computes the position of the mobile terminal by observing time differences among the arrivals of a radio signal transmitted by the mobile terminal at a plurality of observation points, i.e., base stations. The operation of GPS is well known and will not be described further here.
A cellular telephone location system for automatically recording the location of one or more mobile cellular telephones is described in U.S. Pat. No. 5,327,144. The system comprises a central site system operatively coupled to at least three cell sites. Each of the cell sites receive cellular telephone signals and integrate a timing signal common to all the cell sites. The central site calculates differences in times of arrival of the cellular telephone signals arriving among the cell sites and thereby calculates the positions of the cellular telephone producing the cellular telephone signals. The full disclosure of U.S. Pat. No. 5,327,144 is hereby incorporated by reference in its entirety.
A mobile terminal that relies exclusively on the GPS system frequently incurs several problems that can compromise the mobile terminal""s ability to determine its position. For example, buildings can obstruct GPS navigational signals and if the mobile terminal moves inside a building the internal GPS receiver will be unable to calculate position information. Also, the GPS receiver imposes a drain on the mobile terminal""s battery and can shorten the mobile terminal""s standby time.
The external positioning approach relies on an adequate coverage of a cellular positioning system to calculate the position of the mobile terminal. Since the cellular positioning system calculates the position of the mobile terminal by detecting time differences among incoming radio signals transmitted by the mobile terminal, problems will arise if the mobile terminal roams into an area not well served by a cellular positioning system. Also, external positioning is generally expected to be less precise than that provided by an internal GPS receiver due to unknown and variable time-domain jitter and offset introduced by the ambiguities inherent to multipath cellular propagation paths and group delay through the radio frequency (xe2x80x9cRFxe2x80x9d) and intermediate frequency (xe2x80x9cIFxe2x80x9d) paths of external positioning receivers.
In one aspect of the invention, a method is provided for conducting a handoff procedure between a first positioning system and a second positioning system. The position of a mobile terminal is calculated using the first positioning system. When the first positioning is unable to calculate the position of the mobile terminal, the first positioning system is disengaged. The second positioning system is instructed to engage and calculate the position of the mobile terminal. The status of the first positioning system is monitored. When the first positioning system is able to calculate the position of the mobile terminal, the second positioning system is instructed to disengage and the first positioning system is instructed to engage and calculate the position of the mobile terminal.
In one form, the first positioning system is an external positioning system and the second positioning system is an internal positioning system. In another form, the first positioning system is an internal positioning system and the second positioning system is a external positioning system. In yet another form, the external positioning system is an external cellular positioning system, and the internal positioning system is an internal GPS receiver.
In another aspect of the invention, a mobile terminal includes a transceiver, an internal positioning system, and a mobile terminal position controller. The transceiver is used for communicating via wireless signals with the cellular network. The signals include data regarding the position of the mobile terminal as determined by the external positioning system. The mobile terminal position controller determines the availability of the external and internal positioning systems and selects the other of the positioning systems when one positioning system is unavailable.
In one form, the mobile terminal selects the external cellular positioning system when the battery level is low.